This invention relates to a position detecting apparatus in which a light emitting element and a light receiving element are provided on both sides of a rotary board mounted on a rotary shaft, and the angular position of the rotary board, or angle of rotation thereof, is detected from a light reception signal outputted by the light receiving element.
One example of a conventional position detecting apparatus of this type will be described with reference to FIG. 10. As shown in FIG. 10, a rotary board 2 made of glass or plastic material is mounted on a rotary shaft 1, and an opaque layer 3 is formed on the surface of the rotary board 2. The opaque layer 3 is partially removed to form a spiral slit 4. The configuration of the slit 4 is so determined as to meet the following equation: EQU r=a.times..theta.+b (1)
where r is the distance between the center of the rotary shaft 1 and the slit 4, .theta. is the angle measured counterclockwise from the reference angle (0.degree.), and a and b are the constants determined from the configuration of the slit 4.
A light emitting element (not shown), and a light receiving element 5 are arranged both sides of the slit 4, respectively. The light receiving element 5 comprises a position sensing detector (hereinafter referred to as "a PSD", when applicable) the detecting direction of which is a direction of radius of the rotary shaft 1. The light receiving element 5 outputs a light reception signal according to the position of the light beam on it (hereinafter referred to as "a light receiving position", when applicable) which is applied thereto by the light emitting element.
When, in the apparatus thus constructed, the rotary board 2 is rotated, the position of a light beam applied from the light emitting element through the slit 4 to the light receiving element 5 is moved in the direction of radius, i.e., in the detecting direction of the light receiving element 5; that is, the light receiving position on the light receiving element 5 changes with the above-described Equation (1). Hence, the angle of rotation .theta. of the rotary board 2 can be determined by detection of the light reception signal.
However, in the above described conventional position detecting apparatus, the detecting operation is carried out under the condition that the level of the signal reception signal outputted by the light receiving element is proportional to the angle of rotation of the rotary board. Therefore, in the case where, the rotary board is not coaxial with the rotary shaft or there is a play between the rotary shaft and its bearing, the distance (r) from the center of rotation do not meet the above-described Equation (1). Therefore, similarly as in the above-described case, the level of the light reception signal outputted by the light receiving element is not proportional to the angle of rotation of the rotary board, and the angle of rotation of the rotary board detected from the light reception signal is not correct.